This invention relates to methods for integrated circuit isolation, and more particularly to methods for eliminating the bird's beak effect from selective local oxidations of semiconductor electronic devices.
Adjacent elements on an integrated circuit must be electrically isolated from each other if they are to function independently. Therefore, it has long been recognized that the most basic elements of integrated circuit technology include not only active devices and interconnect, but also isolation.
The standard industry process for isolation of CMOS circuits has long been LOCOS ("LOcal Oxidation of Silicon"). In this process, a layer of silicon nitride.sup.1 is patterned to provide an oxidation barrier in locations where active devices are to be formed (i.e. the active regions). The wafer is then placed in an oxidizing environment to grow a thick field oxide (typically at least several thousand Angstroms).sup.2 on the exposed silicon areas. Silicon under the nitride will generally not be oxidized EXCEPT at the edge of the nitride, where lateral diffusion of oxygen under the nitride will cause a laterally tapered shape, known as a "bird's beak," in the resulting structure. FNT .sup.1 Typically this nitride is not directly deposited on silicon, but overlies a thin oxide layer. This nitride/oxide layer (which may include additional layers) is often referred to as the "active stack." FNT .sup.2 The thickness of the field oxide must be sufficient that a conductors atop the field oxide cannot invert the silicon under the oxide (with "channel-stop" or other doping therein), to turn on parasitic transistors.
A typical sequence of the operations involved in a LOCOS technique is shown in FIGS. 1, 2 and 3 of the accompanying drawings. An oxide layer 2 is grown over a substrate 1 of semiconductor silicon. The oxide 2 is then covered with a nitride layer 3, and a subsequent phototechnique step is used to pattern this active stack (oxide 2 and nitride 3), so that the desired isolation locations are exposed, and the desired active areas are covered. A thick layer of silicon oxide is grown to the configuration shown in FIG. 2. During this growth process, oxygen diffuses through the accumulating silicon dioxide layer to the growth interface. Subsequent removal of the nitride yields the structure shown in FIG. 3.
Standard LOCOS technology invariably leaves a bird's beak (the laterally tapered portion) whose length is typically about 85% of the grown oxide thickness. This results in a loss of active area available for making the device. Thus, the design effort devoted to reducing the size of the device, that is the occupied semiconductor area, is frustrated by the area penalty of the bird's beak.
To obviate this serious drawback, the prior art has suggested a number of expedients directed to reducing the size of the bird's beak.
It is well recognized, for instance, that a reduction in length of the beak would also depend on the capability of the nitride to seal the silicon surface and prevent oxygen from diffusing across the nitride/silicon oxide interface. See, e.g., "Sealed-interface local oxidation technology", IEEE TRANS. ELECTRON DEVICES, vol. ED-29, pages 644-561, April 1982; Hui et al., "Electrical Properties of MOS Devices Made With SILO Technology," 1982 IEDM 220ff; both of which are hereby incorporated by reference.
In order to reduce the size of the beak, one might think of subjecting the silicon surface to a direct nitriding process, but such an approach would not be practicable-because it introduces defects from thermal stress.
A second prior art alternative is known by the acronym SWAMI (Sidewall Masked Isolation Technology). See Chiu et al., "The SWAMI--A Defect Free and Near-Zero Bird's-Beak Local Oxidation Process and Its Application in VLSI Technology," 1982 IEDM 224ff, which is hereby incorporated by reference. While being advantageous from several aspects, that technique is beset with problems of sidewall profile control and etching depth. In addition, damage may be caused to the silicon surface during the silicon island etching.
To overcome such problems, the FUROX (FUlly Recessed OXide) method has been proposed as described in "A new fully recessed-oxide field isolation technology for scaled VLSI Circuit fabrication", IEEE ELECTRON DEVICE LETTERS, vol. EDL-7, No. 2, February 1986, pages 124-126, which is hereby incorporated by reference. This process involves at least two separate oxidation steps, of which the first still results in the development of a bird's beak of significant size.
Various other approaches have been proposed, including numerous variations of the aforesaid methods, but all of these still fall short of solving the problems. For example, Jambotkar, "Method for forming recessed Oxide Isolation Islands," 24 IBM TECHNICAL DISCLOSURE BULLETIN 4744-4745(February 1982), which is hereby incorporated by reference, suggests undercutting and backfilling the pad oxide layer in the active stack, and then etching silicon (apparently using an isotropic etch), to achieve a recessed oxide. However, this process does not achieve the objectives of improved density provided by the inventions disclosed herein.
The technical problem addressed by the present invention is to provide a method for eliminating the so-called bird's beak from local oxidations, while overcoming the limitations of current approaches based on prior art techniques.
The solutive idea on which this invention stands is one of only providing a pure nitride/silicon interface in the area where the oxide is to be grown. This avoids the introduction of any defects due to deposition stress into the active areas of the semiconductor device. To achieve this, the pad oxide layer of the "active" stack is undercut and backfilled with nitride, and a recess is then anisotropically etched into the silicon, before the growth of the field oxide.
This has the advantage of providing reduced lateral encroachment on the active area, while also providing improved planarization of the active and isolation regions.